Zhuge Liang is outright the best unit in the game at the moment. The sheer utilities and firepower that he brings to any team are invaluable. With buffs consisting of a 30% crit booster and 30% NP charge for a single target, a 30% defense booster with 10% NP charge, and a 30% attack booster with 10% NP charge (both of the last which are team-wide buffs) Zhuge Liang's importance is unparalleled. That is, until Merlin shows up.

His attack and defense buffs are percentage-based alongside a flat damage cut/flat damage increase, making a Carry + 2 Zhuge team immensely tanky for the first three turns while being capable of firing off the main carry's NP right off the bat. This specific setup will remain one of the best team compositions for difficult content for a long time.

His main problems are that his low-level skills' base cooldowns are huge (7 for Discerning Eye, 8 for the two Tactician skills) and he's only amazing once he unlocks his 3rd Ascension and has access to all of his skills. Thus, Zhuge Liang is a unit that should be regarded as a long-term investment. He also has very poor star generation and low damage, due to being a Caster with a 3-Art-card layout, and if placed in a crit team he will require a good star generator to do the job for him (such as Atalanta or Hassan of the Cursed Arm).

Nonetheless, there is very little reason to avoid investing in Zhuge Liang, as he can trivialize some of the difficult challenges the game can throw at you, and his usefulness will be great no matter how many new units get released.

Strengths

Potent support capability for both offense and defense through his buffs.

Can fit in any type of team composition (Crit, Buster, Art stalling, Art offensive).

Offers amazing survivability and utility with NP for harder content, while being fantastic for normal farming as well.

Helps the team charge NP at a great rate thanks to his skills and 3 Art cards, allowing certain units to fire off their NPs continuously within a short timeframe.

Gains NP quickly thanks to high NP charge per hit and 4 Art cards including NP, making it easy to generate an Art chain.

Weaknesses

Requires a lot of investment to start having a potent impact on the game (at least 3 skills unlocked).

Low damage output due to Casters' innate 0.9 attack multiplier.

NP's stun is chance based, so it's not too reliable.

Very poor at star generation due to low hit count on each card, only 1 Quick card and low star generation rate.

Formalcraft, Projection: Boosting his Art damage output, since he will mostly use his Art cards.

Prisma Cosmos: Allowing him to gain NP without attacking, improving his chances of firing off his NP more often.

Divine Banquet: While worse than both Formalcraft and Projection, it is still a decent alternative.

Holy Shroud of Magdalene: Helps him survive better during battle against male enemies.

Fragment of 2030: As a Support, Fragment of 2030 is basically a wet dream the only thing Zhuge cannot offer is Star generation. Having this Craft Essence makes him a complete package support that gives basically everything to the team.

All three of Zhuge's skills offer too much utility and team support that there is simply no reason not to prioritize leveling up all three. The most straightforward and generalist leveling order is 3 -> 2 -> 1 for the most amount of teamwide support. But overall, as long as you have Zhuge, you should maxing out all three of his skills as soon as possible.

His 3rd skill offers teamwide offensive capability that is very good for early levels grinding, while the 2nd skill adds two layers of defensive buffs that are immensely helpful if your team is full of squishier servants such as Berserker or lower rarity servants. While his 1st skill offers the most Np charge to a single ally, the crit damage buffs are not as good as the defense or attack buffs and can be prioritized last, unless your team is heavily reliant on those skills to give a single unit both NP bar and crit damage, such as Gilgamesh.

Lancelot: Lancelot has one of the worst NP gain rate/hit count combinations in the game, something that Waver can instantly rectify with a 50% NP charge on demand. With the power of Waver, Lancelot is able to reach his high performance design of being a critzerker, hitting hard with his NP, and even harder with his crits. Although Lancelot isn’t very strong on his own, Waver really allows him to shine.

Gilgamesh: In another bout of irony, Waver’s crit damage buff really benefits Gilgamesh, who has the highest rank of Independent Action, as well as strong Buster cards and star weight to make use of said crit damage buff. As with most other servants, Gilgamesh appreciates the 50% NP gauge from Waver’s skills. Additionally, Waver’s defense buff gives Gilgamesh greater durability, making up for Gilgamesh’s lack of defensive skills.

Arash: Similar to Lancelot, a common farming strategy is to set up Arash with a starting NP CE and then let him loose on the first stage or so, allowing another servant to take his place. In his future strengthening quest, Arash receives an NP battery that also restores his health, allowing for more CE flexibility outside or Imaginary Numbers or Kaleidoscope.